1. Field of the Invention
The invention relates to a method of fabricating a thin film transistor, and more particularly to a method of fabricating a thin film transistor including source and drain electrodes comprised of an electrically conductive film composed of aluminum or aluminum alloy.
2. Description of the Related Art
A thin film transistor (TFT) including a thin semiconductor layer such as an amorphous silicon layer and a polysilicon layer is broadly used as a driver or a switching device, for instance, for a liquid crystal display device. A thin film transistor is usually fabricated as MOSFET (Metal Oxide Semiconductor type Field Effect Transistor) including an electrically insulating substrate such as a glass substrate and a thin semiconductor layer formed on the substrate and acting as an active region. A thin film transistor provides an advantage that a driver as mentioned above can be readily fabricated. When a thin film transistor is used in an active matrix type liquid crystal display device, for instance, the thin film transistor is designed to be driven by a driver integrated circuit as a switching device for switching pixels.
A thin film transistor including a thin semiconductor layer composed of poly-semiconductor provides greater carrier mobility than the same provided by a thin film transistor including a thin semiconductor layer composed of amorphous semiconductor, and hence, can operate at a higher rate than a thin film transistor including a thin semiconductor layer composed of amorphous semiconductor. In addition, a switching device for driving pixels and a driver integrated circuit can be fabricated simultaneously on a common substrate in a thin film transistor including a thin semiconductor layer composed of poly-semiconductor. Thus, a thin film transistor including a thin semiconductor layer composed of poly-semiconductor satisfies requirements of a recent liquid crystal display device such as fabrication in a small size or enhancement in a resolution.
FIG. 1 is a cross-sectional view of a conventional thin film transistor having a so-called top-gate structure in which a gate electrode is formed above a thin semiconductor layer.
As illustrated in FIG. 1, the thin film transistor is comprised of an electrically insulating substrate 101 such as a glass substrate, an electrically insulating film 102 formed on the substrate 101, a thin semiconductor layer 103 comprised of a polysilicon film and formed on the electrically insulating film 102, and including a channel region 104 formed substantially centrally of the thin semiconductor layer 103 and source and drain regions 105 and 106 formed around the channel region 104, a gate insulating film 107 formed on the channel region 104, a gate electrode 108 formed on the gate insulating film 107, an interlayer insulating film 109 composed of a silicon oxide film and formed entirely over the substrate 101, and source and drain electrodes 111 and 112 formed in contact holes 110 formed throughout the interlayer insulating film 109 such that the source and drain regions 105 and 106 are exposed through the contact holes 110.
The gate electrode 108 is composed of refractory metal such as chrome (Cr), molybdenum (Mo) or tungsten (W). The source and drain electrodes 111 and 112 are comprised of an electrically conductive film composed of aluminum or aluminum alloy predominantly containing aluminum, because aluminum and aluminum alloy are suitable to small geometry process by etching, and have a small resistance.
FIG. 2 is a cross-sectional view of another conventional thin film transistor having so-called bottom-gate structure in which a gate electrode is formed below a thin semiconductor layer.
The thin film transistor illustrated in FIG. 2 is different from the thin film transistor illustrated in FIG. 1 only in location of the gate electrode 108. The thin film transistor illustrated in FIG. 2 operates under the same principle as that of the thin film transistor illustrated in FIG. 1.
In fabrication of the conventional thin film transistors illustrated in FIGS. 1 and 2, there are carried out many processes such as plasma-etching for patterning a gate metal film such as a chrome film into the gate electrode 108, laser annealing for activating the thin semiconductor layer 103, and plasma-etching of the interlayer insulating film 109 for forming the contact holes 110 therethrough.
These processes cause a problem of crystal defects in the thin semiconductor layer 103, the gate insulating film 107 and/or an interface between the thin semiconductor layer 103 and the gate insulating film 107, because the layers 103 and 107 are damaged by static electricity caused by the plasmas. Such crystal defects further cause a problem of variance in characteristics of a thin film transistor such as a threshold voltage, resulting in deterioration in reliability to a thin film transistor. Accordingly, it is long desired to provide a solution to a problem of crystal defects.
Japanese Patent Application Publication No. 2000-252472A has suggested a method of fabricating such a thin film transistor as mentioned above.
The method includes the steps of forming a thin semiconductor layer comprised of a polysilicon layer, on an electrically insulating substrate, implanting ions of impurity such as boron or phosphorus into a part of the polysilicon layer for forming source and drain regions in the polysilicon layer, forming an interlayer insulating film such as a silicon oxide film over the substrate by plasma-enhanced chemical vapor deposition (CVD), and thermally annealing the substrate at 600 degrees centigrade for an hour for activating the polysilicon film and reducing crystal defects in both the gate insulating film and an interface between the gate insulating film and the polysilicon film.
Japanese Patent Application Publication No. 10-154815A has suggested another method of fabricating such a thin film transistor as mentioned above.
The method includes the steps of forming a thin semiconductor layer comprised of an amorphous silicon film, on an electrically insulating substrate, forming a gate insulating film on the substrate, forming a gate electrode on the gate insulating film above the amorphous silicon film, implanting ions of impurity into a part of the amorphous silicon layer for forming source and drain regions in the amorphous silicon layer, forming a layer composed of one of Cr, Mo, Ta and W, and thermally annealing the layer at 200 to 400 degrees centigrade for a few hours to diffuse Cr, Mo, Ta or W into the source and drain regions for forming a thin silicide layer at surfaces of the source and drain regions.
However, the above-mentioned conventional methods are accompanied with a problem that when source and drain electrodes are comprised of an electrically conductive film composed of aluminum or aluminum alloy predominantly containing aluminum, it would not be possible to prevent crystal defects with the result of variance in characteristics of a thin film transistor.
Specifically, when a gate electrode is formed in the conventional methods of fabricating a thin film transistor, there are carried out many processes such as plasma-etching for patterning a gate metal film such as a chrome film, laser annealing for activating a thin semiconductor layer, and plasma-etching of an interlayer insulating film for forming contact holes therethrough. These processes cause crystal defects in a thin semiconductor layer, a gate insulating film and/or an interface between a thin semiconductor layer and a gate insulating film, because a thin semiconductor layer and a gate insulating film are damaged by static electricity caused by plasmas. This results in variance in characteristics of a thin film transistor such as a threshold voltage.
In the firstly mentioned conventional method of fabricating a thin film transistor, the substrate is thermally annealed at 600 degrees centigrade for an hour subsequently to the formation of source and drain electrodes. Even if the substrate were thermally annealed prior to the formation of source and drain electrodes, crystal defects might be caused when source and drain electrodes are formed. Hence, thermally annealing carried out prior to the formation of source and drain electrodes does not contribute to improvement in crystal defects.
In the secondly mentioned conventional method of fabricating a thin film transistor, the substrate is thermally annealed at 200 to 400 degrees centigrade for a few hours after the formation of source and drain regions, but before the formation of source and drain electrodes. Similarly to the firstly mentioned conventional method, the secondly mentioned conventional method is accompanied with a problem that crystal defects might be caused when source and drain electrodes are formed. As a result, the secondly mentioned conventional method does not contribute to improvement in crystal defects.
Japanese Patent Application Publication No. 8-125022A has suggested a method of fabricating a semiconductor device, including the steps of forming a refractory metal silicide film on a semiconductor substrate in a certain area, forming a silicon film on the refractory metal silicide film, forming an electrode wiring electrically connecting the silicon film to the certain area of the substrate, forming a polysilicon film on an electrically insulating film formed on the substrate, patterning the polysilicon film into a device, and thermally annealing the substrate in oxidizing atmosphere at a predetermined timing after the formation of the polysilicon film.
Japanese Patent Application Publication No. 10-22507A has suggested a method of fabricating a thin film transistor array, including the steps of a first metal film composed of aluminum, on an electrically insulating transparent substrate, forming a second metal film composed of aluminum alloy predominantly containing aluminum, on the first metal film, and forming an anode oxide film by partially anode-oxidizing the second metal film such that a portion of the first and second metal films which acts as a gate or a source electrode remains not removed.
Japanese Patent Application Publication No. 2001-28445A has suggested a method of fabricating a semiconductor device, including the steps of forming a thin film transistor on an electrically insulating film, forming an electrically insulating film over the thin film transistor, and thermally annealing the substrate.